Communicating hydrocephalus causing aqueductal stenosis.

The cited evidence establishes that the pathogenetic sequence of generalized communicating hydrocephalus followed by various degrees of aqueductal stenosis occurs in many mammalian species. The aqueductal stenosis is caused by compression of the midbrain by the expanding cerebral hemisphere. Therefore the sharp separation of communicating and non-communicating hydrocephalus is no longer tenable.